Most thermoplastic polymers and coating compositions are unstable to extended exposure to ultraviolet light source in atmosphere. Thermoplastics and coatings tend to demonstrate unwanted colour changes and reduced mechanical strength upon exposure to UV radiation. The preliminary effect of ultraviolet radiation on polymers is the formation of free radicals on the polymer chain, which react with atmospheric oxygen. This results in the formation of peroxide groups. Furthermore, decomposition of peroxide groups causes formation of carbonyl groups and chain scission. Irradiation in absence of oxygen causes the increase in crosslinking. Ultimately, this reflects on the mechanical properties and the colour of the polymeric materials. In order to prevent or at least retard the damage caused by these factors, stabilizers are added to the plastics.
UV absorbers are compounds that upon addition to the polymers are capable of preventing or retarding the reactions of degradation caused by light energy. 2-Hydroxyphenyl benzotriazoles are one of the most important UV absorbers, which are used commercially. The preparation and use of functional UV absorber in polymers and coatings is well documented.
In order to meet the need for the different UV absorbers for different systems, the reactive intermediates for the synthesis of novel UV absorber are very essential.
Side chain bromination in UV absorbers in the prior art is a three step reaction as reported in the literature [S. Yoshida & O. Vogl, Makromol.Chem., 183, 259(1982); S. Yoshida, C. P. Lillya & O. Vogl, Monatshefte Chem., 113, 603(1982); S. Yoshida, C. P. Lillya & O. Vogl, J.Polym.Sci., Polym.Chem.Ed., 20, 2215(1982)]. Literature reports the similar process via three-step reaction, which includes protection of hydroxyl group, bromination, and then deprotection reaction. However, there is no report of bromination of benzotriazole UV absorbers in the art.
Hydroxyphenyl benzotriazole monomers may be prepared by any method known in the art including those disclosed in U.S. Pat. Nos., 5,104,992; 4,943,637; and 5,097,041. 2-Aryl-2H-benzotriazoles monomers may be prepared by reducing o-nitroazobenzenes through a 2-phenylbenzotriazole-N-oxide intermediate. Reduction of o-nitroazobenzenes to 2-phenylbenzotriazole by zinc in presence of sodium hydroxide is disclosed in U.S. Pat. Nos. 3,018,269; 3,773,751; 4,041,044; and 4,224,451. Aldehyde reducing agents and aromatic ketone catalysts are disclosed in U.S. Pat. No. 4,835,284. All of these patents are incorporated herein by reference.
It is therefore important to obtain bromo functionalized UV absorber is highly reactive and can be used for the synthesis of various functional UV absorbers.